File browser does not show up when operator is executed from a shortcut menu
By looking at the add menu defined in space_sequencer.py it sets layout.operator_context = 'INVOKE_REGION_WIN', this doesn't appear to carry into your menu item.

By setting the operator_context within your menu item method, the sample you provide would appear to work as you expect.

def menu_func_import(self, context):
    self.layout.operator_context = 'INVOKE_REGION_WIN'
    self.layout.operator(ImportSomeData.bl_idname, text="Text Import Operator")